Two kinds of epoxy resin, a soft one with rubber-based additives and an ordinary hard one, are used as electrical insulators and airtight sealants in electrical penetrations in nuclear power plants (NPPs). Knowing how they behave under adverse environments is important for the safe operation of NPPs. In this regard, we gave artificial thermal ageing treatment to the two resins by heating them in air at various temperatures. The resins were then subjected to mechanical, thermal, and dielectric tests. As a result, it has become clear that the soft epoxy resin becomes hard, while its permittivity and loss factor decrease as the heating becomes longer. The glass transition tends to occur at a high temperature in the soft epoxy resin when it was heated. With these changes, the soft resin tends to exhibit similar behaviour in various properties to that of the unaged hard one. On the other hand, the hard one is hardly affected by the simulated thermal ageing. Therefore, both resins can be useable in NPPs.
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